The present invention relates to a method of manufacturing (meth)acrylic acid.
Commercially available (meth)acrylic acid products are manufactured by a manufacturing process that includes a series of a reaction step, an absorbing step, and a refinement step. Specifically, a (meth)acrylic-acid-containing mixed gas prepared by a catalytic gas-phase oxidation reaction in a reactor is introduced to an absorbing column, where (meth)acrylic acid is absorbed using a solvent, such as water. The collected liquid absorbing (meth)acrylic acid is introduced to a refinement step involving various distillation columns in which the (meth)acrylic acid is separated and refined to complete the commercial manufacture of (meth)acrylic acid products.
Here, in the refinement step, a vacuum generator such as an ejector is provided in some kinds of distillation columns used, so as to maintain the interior of the distillation column in vacuum during distillation. As a result, the distillation columns involved in the refinement step produce (meth)acrylic acid, and at the same time discharge a vent gas, that is, a gas containing molecular oxygen introduced for the purpose of inhibiting (meth)acrylic acid from polymerizing.
For these reasons, the vent gas discharged from the distillation columns and their peripherals, including heat exchangers, tanks, and vacuum generators involved in the refinement step, inevitably contain (meth)acrylic acid corresponding to the vapor pressure at a condensation temperature.
Conventionally, the (meth)acrylic acid contained in the vent gas dissolved in a solvent, normally water, used in the ejector was discharged (discarded) in the form of waste water, and the rest in the form of waste gas.
There are other sources for vent gas, including columns and storage tanks (for example, a refined (meth)acrylic acid product storage tank as a product storage tank) used in the absorbing and later steps during which the apparatus is operated at atmospheric or slightly increased pressure. The vent gas from these sources also contain (meth)acrylic acid. Conventionally, the vent gas was, typically, released into the air.
However, as mentioned in the foregoing, the vent gas discharged in the refinement step of manufacture of (meth)acrylic acid contains (meth)acrylic acid. The vent gas, if released into the air or discharged in the form of waste water without being treated, results in a waste of (meth)acrylic acid and reduced yields of (meth)acrylic acid products, and obviously is a cause for environmental pollution too.
Accordingly, the present invention has objects to improve the yields of (meth)acrylic acid products and also eliminate a cause for environmental pollution by efficiently recovering the (meth)acrylic acid contained in the vent gas produced in the refinement step of a (meth)acrylic acid manufacturing process.
In order to accomplish the objects, the inventors of the present invention have diligently conducted researches, and as a result, found that if the (meth)acrylic-acid-containing vent gas produced in the refinement step is recirculated to the absorbing or later step either with no treatment or after condensing the (meth)acrylic acid contained in the vent gas, the (meth)acrylic acid contained in the vent gas is efficiently recovered and also that if the (meth)acrylic acid contained in the vent gas is recirculated, polymers are reduced in the columns used in the absorbing and later steps. The inventors further found that as a result of the foregoing recirculation, the yields of (meth)acrylic acid products are improved, the manufacturing cost of (meth)acrylic acid products is reduced, and a cause for environmental pollution can be eliminated, which has led to the completion of the invention.
In other words, in order to accomplish the objects, the method of manufacturing (meth)acrylic acid in accordance with the present invention is characterized in that it is a method of manufacturing (meth)acrylic acid by a process constituted by a reaction step, an absorbing step, and a refinement step, wherein the (meth)acrylic acid in the vent gas produced in the refinement step is recirculated in the absorbing or later step, that is, in the absorbing step and/or in the refinement step.
According to the foregoing method, the (meth)acrylic acid in the vent gas produced in the refinement step can be efficiently recovered. Further, by recirculating the (meth)acrylic acid contained in the vent gas, polymers are reduced in those absorbing and distillation columns used in the absorbing and refinement steps. Therefore, the yield of (meth)acrylic acid improves, and the manufacturing cost decreases.
In addition, since the (meth)acrylic acid is efficiently recovered, a cause for environmental pollution can be eliminated.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings.